/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
package com.nulldev.util.internal.backport.concurrency9.concurrent;

import java.util.concurrent.Executor;

/**
 * Interrelated interfaces and static methods for establishing flow-controlled
 * components in which {@link Publisher Publishers} produce items consumed by
 * one or more {@link Subscriber Subscribers}, each managed by a
 * {@link Subscription Subscription}.
 *
 * <p>
 * These interfaces correspond to the
 * <a href="http://www.reactive-streams.org/"> reactive-streams</a>
 * specification. They apply in both concurrent and distributed asynchronous
 * settings: All (seven) methods are defined in {@code
 * void} "one-way" message style. Communication relies on a simple form of flow
 * control (method {@link Subscription#request}) that can be used to avoid
 * resource management problems that may otherwise occur in "push" based
 * systems.
 *
 * <p>
 * <b>Examples.</b> A {@link Publisher} usually defines its own
 * {@link Subscription} implementation; constructing one in method
 * {@code subscribe} and issuing it to the calling {@link Subscriber}. It
 * publishes items to the subscriber asynchronously, normally using an
 * {@link Executor}. For example, here is a very simple publisher that only
 * issues (when requested) a single {@code
 * TRUE} item to a single subscriber. Because the subscriber receives only a
 * single item, this class does not use buffering and ordering control required
 * in most implementations (for example {@link SubmissionPublisher}).
 *
 * <pre> {@code
 * class OneShotPublisher implements Publisher<Boolean> {
 * 	private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
 * 	private boolean subscribed; // true after first subscribe
 * 
 * 	public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
 * 		if (subscribed)
 * 			subscriber.onError(new IllegalStateException()); // only one allowed
 * 		else {
 * 			subscribed = true;
 * 			subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
 * 		}
 * 	}
 * 
 * 	static class OneShotSubscription implements Subscription {
 * 		private final Subscriber<? super Boolean> subscriber;
 * 		private final ExecutorService executor;
 * 		private Future<?> future; // to allow cancellation
 * 		private boolean completed;
 * 
 * 		OneShotSubscription(Subscriber<? super Boolean> subscriber, ExecutorService executor) {
 * 			this.subscriber = subscriber;
 * 			this.executor = executor;
 * 		}
 * 
 * 		public synchronized void request(long n) {
 * 			if (!completed) {
 * 				completed = true;
 * 				if (n <= 0) {
 * 					IllegalArgumentException ex = new IllegalArgumentException();
 * 					executor.execute(() -> subscriber.onError(ex));
 * 				} else {
 * 					future = executor.submit(() -> {
 * 						subscriber.onNext(Boolean.TRUE);
 * 						subscriber.onComplete();
 * 					});
 * 				}
 * 			}
 * 		}
 * 
 * 		public synchronized void cancel() {
 * 			completed = true;
 * 			if (future != null)
 * 				future.cancel(false);
 * 		}
 * 	}
 * }
 * }</pre>
 *
 * <p>
 * A {@link Subscriber} arranges that items be requested and processed. Items
 * (invocations of {@link Subscriber#onNext}) are not issued unless requested,
 * but multiple items may be requested. Many Subscriber implementations can
 * arrange this in the style of the following example, where a buffer size of 1
 * single-steps, and larger sizes usually allow for more efficient overlapped
 * processing with less communication; for example with a value of 64, this
 * keeps total outstanding requests between 32 and 64. Because Subscriber method
 * invocations for a given {@link Subscription} are strictly ordered, there is
 * no need for these methods to use locks or volatiles unless a Subscriber
 * maintains multiple Subscriptions (in which case it is better to instead
 * define multiple Subscribers, each with its own Subscription).
 *
 * <pre> {@code
 * class SampleSubscriber<T> implements Subscriber<T> {
 * 	final Consumer<? super T> consumer;
 * 	Subscription subscription;
 * 	final long bufferSize;
 * 	long count;
 * 
 * 	SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
 * 		this.bufferSize = bufferSize;
 * 		this.consumer = consumer;
 * 	}
 * 
 * 	public void onSubscribe(Subscription subscription) {
 * 		long initialRequestSize = bufferSize;
 * 		count = bufferSize - bufferSize / 2; // re-request when half consumed
 * 		(this.subscription = subscription).request(initialRequestSize);
 * 	}
 * 
 * 	public void onNext(T item) {
 * 		if (--count <= 0)
 * 			subscription.request(count = bufferSize - bufferSize / 2);
 * 		consumer.accept(item);
 * 	}
 * 
 * 	public void onError(Throwable ex) {
 * 		ex.printStackTrace();
 * 	}
 * 
 * 	public void onComplete() {
 * 	}
 * }
 * }</pre>
 *
 * <p>
 * The default value of {@link #defaultBufferSize} may provide a useful starting
 * point for choosing request sizes and capacities in Flow components based on
 * expected rates, resources, and usages. Or, when flow control is never needed,
 * a subscriber may initially request an effectively unbounded number of items,
 * as in:
 *
 * <pre> {@code
 * class UnboundedSubscriber<T> implements Subscriber<T> {
 *   public void onSubscribe(Subscription subscription) {
 *     subscription.request(Long.MAX_VALUE); // effectively unbounded
 *   }
 *   public void onNext(T item) { use(item); }
 *   public void onError(Throwable ex) { ex.printStackTrace(); }
 *   public void onComplete() {}
 *   void use(T item) { ... }
 * }}</pre>
 *
 * @author Doug Lea
 * @since 9
 */
public final class Flow {
// CVS rev. 1.32
	private Flow() {
	} // uninstantiable

	/**
	 * A producer of items (and related control messages) received by Subscribers.
	 * Each current {@link Subscriber} receives the same items (via method
	 * {@code onNext}) in the same order, unless drops or errors are encountered. If
	 * a Publisher encounters an error that does not allow items to be issued to a
	 * Subscriber, that Subscriber receives {@code onError}, and then receives no
	 * further messages. Otherwise, when it is known that no further messages will
	 * be issued to it, a subscriber receives {@code
	 * onComplete}. Publishers ensure that Subscriber method invocations for each
	 * subscription are strictly ordered in
	 * <a href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
	 * order.
	 *
	 * <p>
	 * Publishers may vary in policy about whether drops (failures to issue an item
	 * because of resource limitations) are treated as unrecoverable errors.
	 * Publishers may also vary about whether Subscribers receive items that were
	 * produced or available before they subscribed.
	 *
	 * @param <T> the published item type
	 */
	public static interface Publisher<T> {
		/**
		 * Adds the given Subscriber if possible. If already subscribed, or the attempt
		 * to subscribe fails due to policy violations or errors, the Subscriber's
		 * {@code onError} method is invoked with an {@link IllegalStateException}.
		 * Otherwise, the Subscriber's {@code onSubscribe} method is invoked with a new
		 * {@link Subscription}. Subscribers may enable receiving items by invoking the
		 * {@code request} method of this Subscription, and may unsubscribe by invoking
		 * its {@code cancel} method.
		 *
		 * @param subscriber the subscriber
		 * @throws NullPointerException if subscriber is null
		 */
		public void subscribe(Subscriber<? super T> subscriber);
	}

	/**
	 * A receiver of messages. The methods in this interface are invoked in strict
	 * sequential order for each {@link Subscription}.
	 *
	 * @param <T> the subscribed item type
	 */
	public static interface Subscriber<T> {
		/**
		 * Method invoked prior to invoking any other Subscriber methods for the given
		 * Subscription. If this method throws an exception, resulting behavior is not
		 * guaranteed, but may cause the Subscription not to be established or to be
		 * cancelled.
		 *
		 * <p>
		 * Typically, implementations of this method invoke {@code
		 * subscription.request} to enable receiving items.
		 *
		 * @param subscription a new subscription
		 */
		public void onSubscribe(Subscription subscription);

		/**
		 * Method invoked with a Subscription's next item. If this method throws an
		 * exception, resulting behavior is not guaranteed, but may cause the
		 * Subscription to be cancelled.
		 *
		 * @param item the item
		 */
		public void onNext(T item);

		/**
		 * Method invoked upon an unrecoverable error encountered by a Publisher or
		 * Subscription, after which no other Subscriber methods are invoked by the
		 * Subscription. If this method itself throws an exception, resulting behavior
		 * is undefined.
		 *
		 * @param throwable the exception
		 */
		public void onError(Throwable throwable);

		/**
		 * Method invoked when it is known that no additional Subscriber method
		 * invocations will occur for a Subscription that is not already terminated by
		 * error, after which no other Subscriber methods are invoked by the
		 * Subscription. If this method throws an exception, resulting behavior is
		 * undefined.
		 */
		public void onComplete();
	}

	/**
	 * Message control linking a {@link Publisher} and {@link Subscriber}.
	 * Subscribers receive items only when requested, and may cancel at any time.
	 * The methods in this interface are intended to be invoked only by their
	 * Subscribers; usages in other contexts have undefined effects.
	 */
	public static interface Subscription {
		/**
		 * Adds the given number {@code n} of items to the current unfulfilled demand
		 * for this subscription. If {@code n} is less than or equal to zero, the
		 * Subscriber will receive an {@code onError} signal with an
		 * {@link IllegalArgumentException} argument. Otherwise, the Subscriber will
		 * receive up to {@code n} additional {@code
		 * onNext} invocations (or fewer if terminated).
		 *
		 * @param n the increment of demand; a value of {@code
		 * Long.MAX_VALUE} may be considered as effectively unbounded
		 */
		public void request(long n);

		/**
		 * Causes the Subscriber to (eventually) stop receiving messages. Implementation
		 * is best-effort -- additional messages may be received after invoking this
		 * method. A cancelled subscription need not ever receive an {@code onComplete}
		 * or {@code onError} signal.
		 */
		public void cancel();
	}

	/**
	 * A component that acts as both a Subscriber and Publisher.
	 *
	 * @param <T> the subscribed item type
	 * @param <R> the published item type
	 */
	public static interface Processor<T, R> extends Subscriber<T>, Publisher<R> {
	}

	static final int DEFAULT_BUFFER_SIZE = 256;

	/**
	 * Returns a default value for Publisher or Subscriber buffering, that may be
	 * used in the absence of other constraints.
	 *
	 * <p>
	 * <b>Implementation Note:</b> The current value returned is 256.
	 *
	 * @return the buffer size value
	 */
	public static int defaultBufferSize() {
		return DEFAULT_BUFFER_SIZE;
	}
}
